A rare recessively inherited disorder involving an overactive immune system. More specifically, the body becomes infiltrated by large numbers of histiocytes (macrophages) that accumulate in various organs such as the liver, spleen, bone marrow, skin and central nervous system. It usually only occurs in infants and young children. Type 4 is caused by a defect on chromosome 6q24. See also Hemophagocytic lymphohistiocytosis, familial, 4